I am running fsx se, Rex 4 texture direct, and just recently purchased FTX Gold SoCal and things are definitely looking better. However, I don't have FTX Global and wonder : will scenery be much improved if I have both SoCal & Global?

FPS is very important but for now I fly exclusively VFR and scenery is also a high priority.

SCA is an all-in-one package. It comes with it's own textures, landclass, vector data and mesh. So while using Global, Vector and/or OpenLC at the same time does not hurt, it won't have any additional benefit to the area which is already covered by SCA. SCA will overtrump Global/Vector/OpenLC. The same is true for the other FTX regions.

It even comes with it's own custom mesh, so a third party mesh (like FS Genesis, FS Global, Toposim) will only be used if it's of a higher resolution than the one which is included in SCA (10m res.).